1. Field of the Invention
The present invention relates to a multicolored image forming apparatus, such as a copying machine, a facsimile machine and a printer, which can form a multicolored image and a monochromatic image on a recording material.
2. Description of Related Art
Widely used conventional multicolored image forming apparatus is of tandem type, which includes, for the sake of speed up of image output, four image forming units arranged in parallel with each other along a direction in which a recording material is conveyed, each of the four image forming units having an image holder on which electrostatic latent images are formed in correspondence with images having colors, such as cyan (C), magenta (M), yellow (Y) and black (K), peculiar to each image forming unit. With such a multicolored image forming apparatus, a multicolored image can be formed on the recording material by guiding the recording material through each image forming unit only one time.
Such a multicolored image forming apparatus is constructed to form a monochromatic image, in particular, a black-and-white image requiring only black developer, in addition to a multicolored image. Actually, in regard to a multicolored image forming apparatus capable of both multicolored image forming and black-and-white image forming, the black-and-white image forming is generally performed more often than the multicolored image forming.
Since the black-and-white image forming does not require all of the four imaging units, a tandem type multicolored image forming apparatus is constructed to stop rotation of unutilized image holders during black-and-white image forming, to prevent deterioration of unutilized image forming units, in particular, unutilized image holders. This prolongs the life of the image holders, and thereby reduces running costs.
When rotation of image holders which are not involved in image forming is stopped during the black-and-white image forming by the tandem type multicolored image forming apparatus as mentioned above, the image holders whose rotation is stopped should be out of contact with the recording material which is conveyed, to prevent such a situation that a transferred black image is distorted by these image holders whose rotation is stopped. For this purpose, the image holders whose rotation is stopped or a transfer conveyer supporting unit is moved before the black-and-white image forming, to cut the contact between the image holders and the recording material.
To realize both of a non-contact state between the image holders which are not rotated during the black-and-white image forming and the recording material and a contact state between all of the image holders and the recording material during the multicolored image forming, most multicolored image forming apparatuses are constructed to move a transfer material holder unit thereof up and down so as to joint/disjoint the transfer material holder with/from the image holder.
Since the transfer material holder unit is bulky and heavy, extremely large power is needed for a driving unit to move the transfer material holder unit up. However, the transfer material holder unit is seldom moved since the movement is required only when changing an image forming mode from unicolor to multicolor, or from multicolor to unicolor. Consequently, it is uneconomic to use a driving unit having such a large output for seldom required movement of the transfer material holder unit.